Poor power quality and demand fluctuations are costly for electrical utilities and high volume electrical power users. One way that high volume users can reduce their utility costs is to determine their power and demand fluctuations and install equipment that prevents costly deviations. Generally, the equipment used for diagnosing power demand and deviations is bulky, costly, and difficult to use. One leading power industry supplier challenged the conventional approach by developing a compact, notebook PC-based power-demand and line-quality monitoring system. This unique solution is not only light- weight and easy-to-use, it also costs less than most traditional power-test equipment.

Application Summary
Distribution inefficiencies, such as an improper factor, burden a power generation system. Also, customer-induced electrical disturbance can radiate back down the power grid and damage equipment in other grid locations. Many utilities offer rate reductions for customers who improve their power factor and correct their electrical disturbances.

To determine their power factor, users must measure RMS volts, line frequency, real power (watts), and reactive power (VARS), and then compute the ratio of real power to total volt-amps. Then, users may also need to measure other factors such as sag, swell, outage, voltage transient, flicker, and harmonic distortion.

Currently, most test equipment capable of making these data measurements costs in excess of $15,000. In addition to being costly, this equipment is bulky and can be difficult to use because of its complicated front-panel controls.

A major power industry supplier with years of software experience in simulation and analysis developed a smaller, lighter, more cost-effective solution using a portable PC supported by Windows-based software. Because Windows has become a de facto graphical user interface (GUI) standard of almost universal familiarity, it is ideal for the creation of an easy-to-use interface.

Potential Solution
Initially, the supplier evaluated several plug-in A/D boards and rejected all of them because the boards required a “luggable” lunchbox-type computer, which was too bulky to be considered a truly portable solution.

IOtech’s Solution
After rejecting the plug-in board solution, the supplier searched for a portable digitizer. Only one portable digitizer had the speed required for the application — IOtech’s WaveBook. At a mere 3.3 pounds and approximately the same size as a notebook PC, this 1 Msample/sec, 8-channel differential input digitizer provided exceptional value. Its raw speed and high transfer rate allow for cycle-by-cycle over-digitization (128 samples/60 Hz cycle) on each channel, enabling real-time demand analysis. For power-quality measurements (sag, swell, outages, transients, etc.), the suppliers require a 50 kHz per channel rate, which is easily met by the WaveBook’s maximum data sample rate per channel.

Accuracy is also a critical factor for this application and the WaveBook delivers less than 0.1% error. Since voltages and currents are measured on separate channels, the accuracy of the phase measurement is especially important.

A multiplexing digitizer sampling at 1 Msample/sec can easily deliver an acceptable 0.15 degree of error (0.04%) at 60 Hz. However, with higher harmonics, this error becomes unacceptable. For example, in thyristor drive systems, where a 17th harmonic is common, the phase error would increase to an unacceptable 2.57 degrees. At the 50th harmonic, this error would further increase to 7.56 degrees. These problems are completely eliminated by the WaveBook’s simultaneous sample-and-hold option, which reduces the interchannel skew from 1 µs to 100 ns.

Conclusion
The WaveBook’s low-noise, light-weight analog front end, fast 1-MHz A/D, high data transfer rate to PC, and add-in options make it perfect for power analysis and many other applications in the physical test area. It is a digitizer of uncompromising performance.

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